1. Field of the Invention
The present invention relates to synthetic aperture radar (SAR) techniques and more particularly, to a method and apparatus for providing real-time high resolution SAR imagery of a translating ship under the influence of rotational motions from sea state conditions.
2. Prior Art
Prior art airborne synthetic aperture radar systems have the capability for producing a high resolution image of stationary ground targets by virtue of the rotational motion of the SAR with respect to the target field. High resolution in both range and cross-range (azimuth) directions are achieved with short-pulse radar techniques for range resolution, and by digital signal doppler processing for azimuth resolution, so as to produce a two-dimensional image. The application of such known SAR processing techniques to a ship at sea, however, can readily produce distorted and improperly focused ship imagery, precluding ship recognition, because of ship rotational motions brought about by sea state conditions.
Systems for ameliorating some of the adverse effects brought about by ship rotational motions are described in the commonly assigned U.S. patents of Sol Boles, entitled "Range/Azimuth Ship Imaging For Ordnance Control", U.S. Pat. No. 4,546,354, "Range/Azimuth Elevation Ship Imaging for Ordnance Control", U.S. Pat. No. 4,546,355, and "Range/Doppler Ship Imaging For Ordnance Control", U.S. Pat. No. 4,563,686, filed concurrently on June 17, 1982, the disclosures of which are incorporated herein by reference. The above referenced Boles patents all describe methods and apparatus for generating real-time high resolution Synthetic Aperture Radar (SAR) imagery from an airborne platform of a translating ship under the influence of roll, pitch, and yaw motions characteristic of sea state conditions, for the purpose of ship classification and the subsequent delivery of command guided weapons from stand-off ranges. The U.S. Pat. No. 4,546,354 provides an undistorted two-dimensional image of the ship from a direct plot of range versus interferometrically determined azimuth angle of all essential scatterers comprising the ship. The U.S. Pat. No. 4,563,686 on the other hand, provides an improvement in image definition, as well as performance to greater ranges, by displaying range versus doppler, after removal of the "isodop" type distortions (brought about by the non-orthogonality of constant doppler contours with respect to constant range contours), inherent in the formed range/doppler image. The U.S. Pat. No. 4,546,355 patent discloses a unique processing implementation for removing any restrictions due to ship scatterer elevation on the formation of a scaled high resolution image. This advantage is brought about by utilizing elevation angle, as well as azimuth angle, interferometric techniques in conjunction with SAR signal processing techniques.
Whereas the U.S. Pat. No. 4,546,355 patent provides for three scaled high resolution orthogonal image projections of a ship of generalized shape without restrictions, there is need to substantially extend the useful range of such imagery. This is desirable to circumvent the image degradation suffered by plotting angular measurements directly when using the technique underlying the U.S. Pat. No. 4,546,355 system whose location accuracies are known to deteriorate rapidly with increased range due to high signal-to-noise requirements.